Laboratory of Interfaces and Nanosize Systems, Institute of Chemistry, Eötvös Loránd University, Budapest, Hungary.
Langmuir. 2012 Oct 23;28(42):14944-53. doi: 10.1021/la302998k. Epub 2012 Oct 11.
Molecular dynamics simulations of the adsorption layer of five different surfactant molecules, i.e., pentyl alcohol, octyl alcohol, dodecyl alcohol, sodium dodecyl sulfate, and dodecyl trimethyl ammonium chloride are performed at the free surface of their aqueous solution at two surface densities, namely 1 and 4 μmol/m(2) at 298 K. The results are analyzed in terms of the two-dimensional single molecule dynamics, in particlular, lateral diffusion of the surfactants at the liquid surface, in order to distinguish between two possible adsorption scenarios, namely the assumptions of localized and mobile surfactants. The obtained results, in accordance with the dynamical nature of the liquid phase and liquid surface, clearly support the latter scenario, as the time scale of lateral diffusion of the surfactant molecules is found to be comparable with that of the three-dimensional diffusion of water in the bulk liquid phase. The mechanism of this lateral diffusion is also investigated in detail by calculating binding energy distribution of the water molecules in the first hydration shell of the surfactant headgroups and that of the nonfirst shell surface waters, and by calculating the mean residence time of the water molecules in the first hydration shell of the surfactant headgroups. This time is found to be at least an order of magnitude smaller than the characteristic time of the lateral diffusion of the surfactants, revealing that surfactant molecules move without their first shell hydration water neighbors at the surface.
在 298 K 下，对五种不同表面活性剂分子（戊醇、辛醇、十二醇、十二烷基硫酸钠和十二烷基三甲基氯化铵）在其水溶液的自由表面上的吸附层进行了分子动力学模拟，表面密度分别为 1 和 4 μmol/m(2)。结果从二维单分子动力学的角度进行了分析，特别是表面活性剂在液相中的横向扩散，以区分两种可能的吸附情况，即局部化和可移动表面活性剂的假设。所得到的结果与液相和液面相符合，明确支持后者的情况，因为表面活性剂分子的横向扩散时间尺度与水在体相中的三维扩散时间尺度相当。通过计算表面活性剂头基第一水合层中水分子的结合能分布和非第一层表面水的结合能分布，以及计算表面活性剂头基第一水合层中水分子的平均停留时间，详细研究了这种横向扩散的机制。这个时间至少比表面活性剂横向扩散的特征时间小一个数量级，表明表面活性剂分子在表面上没有第一壳层水化水邻居的情况下移动。
